1. Field of the Invention
The present invention relates to a global position receiver capable of receiving both GPS and GLONASS signals and in particular to using a hybrid bit extraction technique to provide both frequency error correction as well as bit error correction.
2. Related Art
GPS (global positioning system) and GLONASS (global navigation satellite system) are radio-based satellite systems in operation today. To provide global coverage, GPS uses between 24-32 satellites. Assuming the minimum number of 24 satellites, 4 satellites are deployed in each of six orbits. The six orbital planes' ascending nodes are separated by 60 degrees. In this configuration, a minimum of six satellites should be in view from any given point at any time. To provide global coverage, GLONASS includes 24 satellites, wherein 21 satellites can be used for transmitting signals and 3 satellites can be used as spares. The 24 satellites are deployed in three orbits, each orbit having 8 satellites. The three orbital planes' ascending nodes are separated by 120 degrees. In this configuration, a minimum of five satellites should be in view from any given point at any time.
Both GPS and GLONASS broadcast two signals: a coarse acquisition (C/A code) signal and a precision (P code) signal. In general, global position devices, called receivers herein, lock onto the C/A transmission and not the P transmission. The P transmission is much longer than the C/A transmission and therefore is impractical to lock onto, e.g. by using synchronization. Once a lock is established via C/A transmission, the C/A transmission itself can provide a quick P lock.
The C/A codes for GPS and GLONASS, which can be generated as a modulo-2 sum of two maximum length shift register sequences, are selected for good cross-correlation properties. Each GPS satellite transmits its own unique C/A code, which has an identifiable pseudo-random noise code number (PRN#). In contrast, each GLONASS satellite transmits the same C/A code, and is identified by its channel number (CHN#).
The C/A code includes navigation data, which provides information about the exact location of the satellite, the offset and drift of its on-board atomic clock, and information about other satellites in the system. In GPS, the C/A format for the navigation data includes words, frames, and sub-frames. The words are 30 bits long; ten words form one sub-frame; and five sub-frames form one frame. In GPS, the C/A code is 1023 bits long, is transmitted at 1.023 Mbps, and therefore has a repetition period of 1 ms. In GLONASS, the C/A format is strings, wherein each string includes 1.7 sec of navigation data and 0.3 sec of a time mark sequence. Notably, the C/A code in GLONASS is 511 bits long, is transmitted at 511 kbps, and therefore has the same code repetition period (i.e. 1 ms) as GPS.
Differential detection of GPS bits is known by those skilled in the art of global positioning. For example, U.S. Publication 2008/0143594 describes an exemplary differential detection technique including a sliding window. FIG. 1 illustrates a first sliding window 101 of past N×20 ms I/Q samples and a second window 102 of further M×20 ms I/Q samples. The data bit decoding integrates both windows 101 and 102, where each 20 ms I/Q sample corresponds to one data bit and N and M are integers. The coherent integration for window 101 is performed by using the previously decoded bits to demodulate and coherently integrate the demodulated I/Q samples. Thus, decision feedback is used to demodulate and coherently integrate the I/Q samples within window 101. In window 102, multiple symbol differential detection is used to demodulate and coherently integrate the I/Q samples.
Differential detection is then used to decode the data bit (location 103) based on the coherent integration of the previous N×20 ms I/Q samples and the future M×20 ms I/Q samples. This decoding may be performed by computing a phase angle transition between the past and future coherent integrations. In one embodiment, this phase angle transition may be performed by taking a dot product of the two integrations, e.g. by multiplying the future integration against the conjugate of the past integration and taking the real part. After the data bit is decoded, the previous and future sliding windows are moved forward by 20 ms to decode the next data bit. Note that the length of the sliding windows may be changed based on the error rate of the data bits. For example, the length of the sliding windows may be shortened when the signal is weak and the error rate high, whereas the length may be lengthened when the error rate of the data bits becomes small.
Unfortunately, with differential detection, a single decoding error may cause all the following bits to be reversed. However, if one-bit error correction is applied to an erroneous packet by flipping the estimated erroneous bit and the following bits, it may induce false success due to the limitation of a downstream parity check. Specifically, a parity check is useful for detecting two or fewer bit errors. Thus, when considering the 30-bit GPS word, which contains 24 bits of data and 6 bits of parity check, if the bit flipping is performed at the wrong place and generates more than two bit errors, then a parity check may trigger a “pass” because it cannot detect more than two errors.
Therefore, a need arises for a detection system and method that can detect global positioning bits. A further need arises for a detection system and method that can detect both GPS and GLONASS bits with minimum errors and false success rate.